1. Field of the Invention
The present invention generally relates to an optical scanning apparatus (multi-beam scanning apparatus) for using a plurality of optical beams projected from an illuminant to simultaneously scan a surface to be scanned in the main scanning direction, an illuminant apparatus prepared for the optical scanning apparatus, and an image forming apparatus that uses the optical scanning apparatus as an optical writing system such as a laser color printer, a digital color copier, a laser plotter and a laser facsimile.
2. Description of the Related Art
In order to improve the recording speed of an optical scanning apparatus that is used as an optical writing system in an image forming apparatus, there is an approach in which the rotational speed of a polygon mirror, which serves as a deflecting part, is increased. However, this approach has limited improvement with respect to the recording speed if motor durability, motor noise, motor vibration, laser modulation speed and the like are taken into account. Consequently, optical scanning apparatuses for simultaneously radiating a plurality of optical beams in order to record a plurality of lines have been proposed.
Some multi-beam illuminant apparatuses that can radiate a plurality of laser beams as an illuminant part of these optical scanning apparatuses have been also proposed. As such a multi-beam illuminant apparatus, there is a multi-beam semiconductor laser (for instance, a semiconductor laser array) that contains a plurality of illuminant points (illuminant channels). Regarding such a semiconductor laser array, however, it is extremely difficult to increase the number of the illuminant channels in course of fabrication thereof. In addition, it is hard to eliminate influences on thermal/electronic crosstalk and realize a short wavelength with respect to the semiconductor laser array. For these reasons, the semiconductor array is currently considered to be expensive as an illuminant part of an optical scanning apparatus.
On the other hand, a single-beam semiconductor laser is widely used in various industries because the wavelength of the single-beam semiconductor laser can be easily shortened and the fabrication cost thereof is affordable even in the current technology level.
An illuminant apparatus and a multi-beam scanning apparatus use such a single-beam semiconductor laser or the above-mentioned multi-beam semiconductor laser as an illuminant. In this case, the illuminant apparatus and the optical scanning apparatus need to have a beam synthesizing part for synthesizing a plurality of laser beams. With this background, a large number of illuminant apparatuses and optical scanning apparatuses for using a beam synthesizing part to synthesize a plurality of laser beams have been proposed.
However, when such a beam synthesizing part is used to synthesize a plurality of laser beams, a single-beam semiconductor laser has some problems in comparison with a semiconductor laser array in that environmental variations and time passage make alignment of beam spots on a surface to be scanned, for instance, the beam pitch and the scanning line interval, unstable.
Japanese Laid-Open Patent Application No. 2000-227563 discloses a multi-beam scanning optical device for using a beam synthesizing prism to synthesize optical beams emitted from a plurality of illuminants. In this multi-beam scanning optical device, projection directions of the optical beams are adjusted by shifting the beam synthesizing prism along a light path and adjusting gradient of the beam synthesizing prism in the main scanning section or the subscanning section. As a result, it is possible to adjust positions of beam spots on a surface to be scanned.
Japanese Laid-Open Patent Application No. 10-215351 discloses a light beam scanner for using a beam synthesizing prism to synthesize optical beams emitted from a plurality of illuminants. In this light beam scanner, projection directions of the optical beams are adjusted by shifting a cylindrical lens for forming a line image on a reflection surface of a polygon mirror in the subscanning direction. As a result, it is possible to adjust positions of beam spots on a surface to be scanned.
Japanese Laid-Open Patent Application No. 09-189873 discloses a device for scanning multi-beam for using a half mirror to synthesize optical beams emitted from a plurality of illuminants. In this device for scanning multi-beam, projection directions of the optical beams are adjusted by adjusting gradients of a galvanomirror in a light path and an illuminant apparatus. As a result, it is possible to adjust positions of beam spots on a surface to be scanned.
Japanese Laid-Open Patent Application No. 10-282531 discloses an optical deflector for deflecting a beam laser through varying refractive index of an electrooptic material (lithium niobate and so on) with electrooptic effect.
Japanese Laid-Open Patent Application No. 2000-003110 discloses an image forming device and control method thereof. In this image forming device, a light path deflecting element (a liquid crystal element) is used to adjust a scanning position on a surface to be scanned, and the pitch irregularity of scanning lines, which is caused by the rotational irregularity of a photoreceptor drum, is corrected. In addition, the image forming device has a detecting part for detecting the rotational speed of the photoreceptor drum.
Japanese Laid-Open Patent Application No. 2000-047214 discloses an optical path variable device and an image forming device. A light path deflecting element formed of a plurality of liquid crystal layers is provided to an image forming apparatus for forming an image. Like the image forming device according to Japanese Laid-Open Patent Application No. 2000-003110, the optical path variable device and the image forming device intends to correct the pitch irregularity of scanning lines caused by the rotational irregularity of a photoreceptor drum.
Many of existing inventions including the multi-beam scanning optical device according to Japanese Laid-Open Patent application No. 2000-227563, the light beam scanner according to Japanese Laid-Open Patent application No. 10-215351, and the device for scanning multi-beam according to Japanese Laid-Open Patent application No. 09-189873 use some mechanical systems to deflect a light path of an optical beam and adjust alignment of a beam spot on a surface to be scanned.
In the case where such a mechanical system is used to adjust the light path, however, since the number of parts thereof inevitably increases, the part increase not only undermines reliability and duration of the whole system but also increases the system size. In addition, there is a probability that hysteresis caused by backlash and others, vibration, noise and heat arise.
Also, some methods are presented along another approach. One method utilizes variations of the prism refractive index caused by electrooptical effect such as the optical deflector according to Japanese Laid-Open Patent application No. 10-282531. Also, in another method, an optical beam is deflected through diffraction caused by an acoustooptic element. However, since a high driving voltage is required to implement these methods, an apparatus thereof becomes complicated and includes risk of heat generation. Thus, these methods are considered to be impractical.
In the image forming device according to Japanese Laid-Open Patent application No. 2000-003110 and the optical path variable device and image forming device according to Japanese Laid-Open Patent application No. 2000-047214, such an image forming apparatus uses a liquid crystal element as a light path deflecting element so as to correct the pitch irregularity of scanning lines due to the rotational irregularity of a photoreceptor drum or more accurately superpose image information between photoreceptor drums in a tandem type optical scanning apparatus.
In a tandem type full color image forming apparatus, four photoreceptor drums corresponding to the four colors: cyan (C), magenta (M), yellow (Y) and black (K) are provided in a tandem form along the carrier surface of an intermediate transferring belt. When a beam scanning apparatus scans an image by using optical beams corresponding to the individual photoreceptors, electrostatic latent images are formed on the circumferential surfaces of the photoreceptor drums. The electrostatic latent images are developed by using the corresponding color toners. The color toner images are carried by the intermediate transferring belt, and then a color image is created by sequentially transferring the color toner images.
In the beam scanning apparatus, a scanning part is rotationally driven at a predetermined rotational speed by a polygon motor and so on. A line synchronization signal generating part detects an optical beam from the beam scanning apparatus at a predetermined position and then generates a line synchronization signal. Synchronously with the line synchronization signal, an optical beam is modulated in accordance with the image signal, and then the image is written for each line. An intermediate transfer reference signal generating part detects a mark on an intermediate transfer body at a predetermined position and then generates an intermediate transfer reference signal. Synchronously with the intermediate transfer reference signal, toner images corresponding to the individual colors are formed on the photoreceptor.
However, the intermediate transfer reference signal and the line synchronization signal are not synchronized with each other in the color image forming apparatus. Thus, as the number of optical beams increases, the phase difference between the intermediate transfer reference signal and the line synchronization signal tends to become larger. When the difference between the image writing start positions becomes large with respect to the subscanning direction, there arises color smear (misalignment of the individual toner images) and the resulting color image deteriorates.
Japanese Laid-Open Patent Application No. 10-239939 discloses a color image forming device that can eliminate the above-mentioned problems. The color image forming apparatus has a correcting part correcting a color smear by selecting the first optical beam for writing an image on a photoreceptor among a plurality of optical beams in accordance with phase relation between an intermediate transfer reference signal and a line synchronization signal and adjusting image writing start positions of individual colors with respect to the subscanning direction. However, even if this method is applied to color image forming apparatuses, there still is a probability that at most one line of misalignment arises with respect to the image writing start positions.
When a liquid crystal element is used to deflect a light path by an infinitesimal angle, it is necessary to coincide (parallel) an optical axis of the liquid crystal element with a plane of polarization of an optical beam having linear polarization. In order to enhance shading characteristic (light intensity distribution) on a surface to be scanned of a photoreceptor, a semiconductor laser (single beam) is inclined in the optical axis direction. Also, in order to set a subscanning beam pitch on a surface to be scanned to a predetermined value in a multi-beam scanning apparatus using a semiconductor laser array as an illuminant part thereof, the semiconductor laser array is inclined in the optical axis direction in accordance with the optical magnification. In these cases, there is a probability that the deflection direction of the light path does not always coincide with the polarization plane of the optical beam. If the deflection direction of the light path does not coincide with the polarization plane of the optical beam, it is impossible to effectively deflect the light path by means of the liquid crystal element.